Introduction-. 
vn 
SPORANGIA OF FERNS, SHOWING 
DISTRIBUTION OF SPORES. 
the sori have some definite relation to the veins of the frond, the receptacle being usually 
situated at the extremity or on the back of a vein ; but this is not always the case. The 
most familiar form of sorus is circular, as is seen in the Male Fern and Polypody ; frequently 
they are oval or oblong, as in many Aspleniums ; at other times elongated and linear, as in 
the Hart’s-tongue. When they occupy the margin of the frond 
or its under-surface, as in the Bracken, several sori are confluent 
to form the long line of fructification ; and in some foreign ferns 
(Acrostic/tum, Platycerium ) the sori occupy the whole under (and even 
upper) surface of the frond with a continuous layer. In the Filmy 
Ferns the sori are marginal and quite peculiar, the receptacle 
extending beyond the edge of the frond, and being, in fact, the 
end of a vein. In many Ferns which have 
special fertile fronds the fructification is often 
greatly crowded, so that the leafy portion 
becomes contracted and curled up so as to 
be scarcely observable, and this former seems to constitute by itself a spike 
or raceme of fruit ; this is noticeable in the Parsley Fern and the Royal 
Fern amongst European species, and is still more remarkable in some 
exotic ones. 
The sporangia, or capsules, are very characteristic in form, and of a 
remarkable constitution. They are delicate little sacks, with thin, semi- 
transparent walls, and are attached to the receptacle either by a slender 
stalk or are sessile. In shape they are usually 
oval, blunt, and a little lop-sided, in consequence 
of the existence of a ring, which runs vertically 
round one side and over the top, which is called the annulus. 
is a provision of Nature for causing the bursting of the sporangium, 
which happens by the contraction (from drying up) of the ring, and 
the consequent rupture transversely of the wall on the opposite side, 
which allows the escape of the contained spores. In the Filmy Ferns, 
however, the ring is horizontal and extends round the whole sporangium, 
hence the splitting here is vertical ; the splitting is also vertical in 
Osmunda, where the ring, however, is very faint and imperfect. In 
Ophioglossum and Botrychium, what are still called the sporangia are 
wholly different from those above described, and are modified portions 
of the frond, and the spores are formed from changes taking place in 
its inner tissue. All these points are considered by the botanist of 
fundamental importance. 
Another organ connected with the sori must also be examined, for 
though of less intrinsic value to the plant, it is principally employed in 
the classification of ferns generally adopted. This is the indusium, or 
involucre, which is a membranous body of various form, which covers to 
a greater or less extent the sorus. It is very readily observed in most ferns, as our Male 
Fern, and as a rule is of much the same form as the sorus, with which it is connected. In a 
number of species, however, it does not exist at all, as may be seen in the Polypody, in which 
the sori are naked at all periods of their existence. In some cases the involucre is difficult to 
SPORANGIA OF MALE 
FERN. 
This 
UNDERSIDE OF PINNA OF 
MALE FERN. 
